halloysite nanotubes/natural rubber (hnts/nr...

HALLOYSITE NANOTUBES/NATURAL RUBBER

(HNTs/NR) NANOCOMPOSITES: PREPARATION

AND PROPERTIES

SITI ZULIANA SALLEH

UNIVERSITI SAINS MALAYSIA

2012

HALLOYSITE NANOTUBES/NATURAL RUBBER

(HNTs/NR) NANOCOMPOSITES: PREPARATION AND

PROPERTIES

By

SITI ZULIANA SALLEH

Thesis submitted in fulfillment of the requirements

for the degree of

Master of Science

April 2012

DECLARATION

I declare that the content presented in this dissertation is own my work which was

done at Universiti Sains Malaysia unless onformed otherwise. This dissertation has

not been previously submitted for any other degree.

Saya isytiharkan bahawa kandungan yang dibentangkan di dalam disertasi ini adalah

hasil kerja saya dan dijalankan di Universiti Sains Malaysia kecuali dimaklumkan

sebaliknya. Disertasi ini juga tidak pernah disertakan untuk ijazah yang lain

sebelumnya.

Signature:

Candidate‟s Name: Siti Zuliana Salleh

Date:

Witness:

Signature:

Supervisor‟s Name: Prof. Hanafi Ismail

Date:

ii

ACKNOWLEDGEMENT

Bismillahirrahmanirrahim,

My greatest gratitude to Allah S.W.T, for his grace for giving me an

opportunity and enables me to complete my research. I am deeply grateful to my

supervisor, Prof. Hanafi Ismail, who has supported me throughout my project with

his patience and encouragement. I would like to acknowledge the advice and

guidance of Assoc. Prof. Dr. Zulkifli Ahmad, my co-supervisor.

I am also grateful to Universiti Sains Malaysia (USM) for the financial

support under USM fellowship. I am sincerely appreciates School of Materials and

Mineral Resources and Institute of Postgraduate Studies (IPS) for giving me an

opportunity for me to carry out and complete my research studies here.

I am extremely grateful to my mother; Che Thum Sulong and my family by

encourage me to complete my study. My appreciation is extended to staff and

technician of School of Materials and Mineral Resources, USM for their kindness

and assistance. Special thanks to technicians namely; Mr. Gnanasegaran a/l N.B.

Dorai, Mr. Sofi, Mr. Faizal, Mr. Mohamad Hassan, Mr. Rashid, Miss Mahani, Mr.

Saarani, Mr. Sharil, and Mr. Mokhtar for helping me in various laboratories work.

Lastly, I offer my regards and blessings to all my friends and those who supported

me in the completion of the project.

iii

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENT ii

TABLE OF CONTENTS iii

LIST OF TABLES vii

LIST OF FIGURES viii

LIST OF SYMBOLS xii

LIST OF ABBREVIATIONS xiv

ABSTRAK xvi

ABSTRACT xvii

CHAPTER 1 – INTRODUCTION

1.1 Introduction 1

1.2 Problem statement 3

1.3 Research objectives 5

1.4 Scope of research 5

CHAPTER 2 – LITERATURE REVIEW

2.1 Introduction to rubber 7

2.1.1 Natural rubber 10

2.1.2 Epoxidized natural rubber (ENR) 11

2.2 Rubber compound 13

2.2.1 Mixing process 13

2.2.2 Forming 15

2.2.3 Vulcanization 15

iv

2.3 Compounding ingredients 17

2.3.1 Vulcanization agents 17

2.3.2 Activators 19

2.3.3 Accelerators 19

2.3.4 Antidegradants 20

2.4 Fillers 22

2.4.1 Halloysite nanotubes (HNTs) 23

2.4.2 Carbon Black (CB) 26

2.5 Polymer nanocomposites 28

2.6 Clay/rubber nanocomposites 32

CHAPTER 3 – MATERIALS, METHODOLOGY AND CHARACTERIZATION

3.1 Materials 41

3.1.1 Matrix 41

3.1.2 Filler 41

3.1.3 Compound ingredients 42

3.2 Methodology 42

3.2.1 Formulation rubber compounds 42

3.2.2 Mixing process 44

3.2.3 Curing characteristics 45

3.2.4 Compression molding 45

3.3 Testing and characterization 46

3.3.1 Tensile test 46

3.3.2 Swelling percentage test 46

3.3.3 Fatigue life test 46

v

3.3.4 Thermogravimetric analysis (TGA) 47

3.3.5 Morphological properties test 47

CHAPTER 4 – RESULTS AND DISCUSSION

4.1 The effect of different HNTs loading in two types of rubber matrixes

(SMR L and ENR 50)

48


4.1.2 Tensile properties 54

4.1.3 Swelling percentage 59

4.1.4 Fatigue life 61


4.1.6 Morphological properties 66

4.1.6.1. Tensile fractured surfaces 66

4.1.6.2. Fatigue fractured surfaces 68

4.2 The effect of halloysite nanotubes (HNTs)/carbon black (CB) hybrid

fillers filled SMR L nanocomposites

71







4.2.6.1 Tensile fractured surfaces 81

4.2.6.2 Fatigue fractured surfaces 82

vi

4.3 The comparison of different mixing methods of HNTs/SMR L

nanocomposites

84







4.3.6.1. Tensile fractured surfaces 94

4.3.6.2. Fatigue fractured surfaces 95

CHAPTER 5 – CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

5.1 Conclusion 98

5.1.1 The effect of different HNTs loading in two types of rubber

matrixes (SMR L and ENR 50)

98

5.1.2 The effect of halloysite nanotubes (HNTs)/carbon black

(CB) hybrid fillers filled SMR L nanocomposites

99

5.1.3 The comparison of different mixing methods of

HNTs/SMR L nanocomposites

100

5.2 Suggestions for future work 101

REFERENCES 103

APPENDICES 117

List of Publications (Articles/Manuscripts) 117

Paper Presentations (Local/International) 117

vii

LIST OF TABLES

Captions Page

Table 2.1 Technical Grades of Natural Rubber 11

Table 2.2 Various ingredients in rubber compounds 13

Table 2.3 CV, Semi-EV and EV vulcanization systems 18

Table 2.4 Clay minerals 24

Table 2.5 Types of carbon black used in tires 28

Table 3.1 The specifications of halloysite 42

Table 3.2 The formulation for Series (1) and (3) 43

Table 3.3 The formulation for Series (2) 43

Table 4.1 Decomposition temperatures at various weight losses for

both nanocomposites (HNTs/SMR L and HNTs/ENR 50

nanocomposites)

66

Table 4.2 Decomposition temperatures at various weight losses for

HNTs/CB/SMR L nanocomposites

81

Table 4.3 Decomposition temperature at various weight losses for

both nanocomposites (mechanical and solution mixing

nanocomposites)

93

viii

LIST OF FIGURES

Captions Page

Figure 1.1 Summary for scope of research in this study 6

Figure 2.1 General chemical structure of cis-1, 4-polyisoprene

(natural rubber)

8

Figure 2.2 General chemical structure of epoxidized natural rubber

(ENR)

12

Figure 2.3 Chemical structure of 2, 2 methylene-bis-(4-methyl-6-tert-

butylphenol) (BKF)

21

Figure 2.4 General structure of para-phenylenediamine (PPD) 22

Figure 2.5 Schematic diagrams of (a) the crystalline structure of

halloysite (10Å), and (b) the structure of a halloysite

nanotube

25

Figure 2.6 Schematic representations of the preparation of

nanocomposites via (a) mechanical and (b) solution

mixing methods

31

Figure 2.7 Transmission electron microscopy images for various

morphologies structure for polymer/clay nanocomposites

(a) microcomposite, (b) intercalated and (c) exfoliated

32

Figure 4.1 The effect of HNTs loading on scorch time (ts2) of

HNTs/SMR L and HNTs/ENR 50 nanocomposites

51

Figure 4.2 The effect of HNTs loading on cure time (t90) of


52

Figure 4.3 The effect of HNTS loading on maximum torque (MH) for


53

Figure 4.4 Tensile strength of HNTs/SMR L and HNTs/ENR 50

nanocomposites with various HNTs loading

57

Figure 4.5 Elongation at break (Eb) of HNTs/SMR L and

HNTs/ENR 50 nanocomposites with various HNTs

loading

58

Figure 4.6 Modulus at 100% and 300% elongation (M100 and M300

respectively) for HNTs/SMR L and HNTs/ENR 50


59

ix

Figure 4.7 Swelling percentage at various HNTs loading filled ENR

50 and SMR L nanocomposites

60

Figure 4.8 The effect of HNTs loading on fatigue life of HNTs/SMR

L and HNTs/ENR 50 nanocomposites

62

Figure 4.9 Weight loss (%) for HNTs/SMR L nanocomposites at

various HNTs loading

65

Figure 4.10 Weight loss (%) for HNTs/ENR 50 nanocomposites at


65

Figure 4.11 SEM images for various HNTs; 10 phr in (a) SMR L and

(b) ENR 50, 20 phr in (c) SMR L and (d) ENR 50, and 40

phr in (e) SMR L and (f) ENR 50 nanocomposites

67

Figure 4.12 Typical fatigue failure surface for HNTs/SMR L

nanocomposites

68

Figure 4.13 Fatigue failure surface for 10 phr of (a) HNTs/SMR L and

(b) HNTs/ENR 50 nanocomposites at 100x magnification

69

Figure 4.14 Fatigue failure surface for 20 phr of filler loading (a)

HNTs/SMR L and (b) HNTs/ENR 50 nanocomposites at

100x magnification

70

Figure 4.15 Scorch time and cure time for HNTs/CB hybrid with

varies filler loading ratio for SMR L nanocomposites

72

Figure 4.16 Maximum torque (MH) for HNTs/CB hybrid with varies

filler loading ratio for SMR L nanocomposites

73

Figure 4.17 Tensile strength of HNTs/CB/SMR L nanocomposites

with various HNTs loading

74

Figure 4.18 Elongation at break (Eb) of HNTs/CB/SMR L


76

Figure 4. 19 Modulus at 100% of elongation (M100) of

HNTs/CB/SMR L nanocomposites with various HNTs

loading

77

Figure 4.20 Modulus at 300% of elongation (M300) of

HNTs/CB/SMR L nanocomposites with various HNTs

loading

77

Figure 4.21 Swelling (%) of HNTs/CB/SMR L nanocomposites with


78

x

Figure 4.22 Fatigue life of HNTs/CB/SMR L nanocomposites with


79

Figure 4.23 Weight loss (%) for HNTs/CB/SMR L nanocomposites 81

Figure 4.24 SEM images for various HNTs/CB ratio filled SMR L

nanocomposites at 10Kx magnification where (a) 40/0, (b)

30/10, (c) 20/20 and (d) 04/0 (phr/phr) HNTs/CB ratios

82

Figure 4.25 SEM images of fatigue fractured surfaces for various

HNTs/CB ratio filled SMR L nanocomposites at 30x

magnification (a) 40/0, (b) 20/20 and (c) 0/40 (phr/phr)

HNTs/CB

83

Figure 4.26 Scorch and cure times of HNTs/SMR L nanocomposites

with various HNTs loading for two different mixing

methods

85

Figure 4.27 Maximum torque (MH) of HNTs/SMR L nanocomposites

with various HNTs loading for two different mixing

methods

86

Figure 4.28 Tensile strength of HNTs/SMR L nanocomposites with

various HNTs loading for two different mixing methods

87

Figure 4.29 Elongation at break (Eb) of HNTs/SMR L

nanocomposites with various HNTs loading for two

different mixing methods

88

Figure 4.30 Modulus at 100% and 300% of elongation (M100 and

M300) of HNTs/SMR L nanocomposites with various

HNTs loading for two different mixing methods

89

Figure 4.31 Swelling (%) of HNTs/SMR L nanocomposites with


90

Figure 4.32 Fatigue life of HNTs/SMR L nanocomposites with


91

Figure 4.33 Weight loss (%) of HNTs/SMR L nanocomposites with


92

Figure 4.34 SEM images for 10 phr of HNTs filled SMR L

nanocomposites via (a) mechanical (b) solution mixing

methods, 20 phr of HNTs filled SMR L nanocomposites

via (c) mechanical (d) solution mixing methods and 40

phr of HNTs filled SMR L nanocomposites via (e)

mechanical (f) solution mixing methods

95

xi

Figure 4.35 Typical SEM images for fatigue fractured surfaces for 20

phr of HNTs filled SMR L nanocomposites via (a)

mechanical (b) solution mixing methods

96

Figure 4.36 SEM images for fatigue fractured surfaces for 10 phr of

HNTs filled SMR L nanocomposites via (a) mechanical

(b) solution mixing methods

97

xii

LIST OF SYMBOLS

Symbol Description

% Percentage

°C Degree Celsius

Al2O3 Alumina (aluminium oxide)

CaCO3 Calcium carbonate

cm Centimeter

cpm Cycles per minute

dNm Deci Newton Meter

Eb Elongation at break

Fe2O3 Iron (III) oxide

g Gram

K2O Potassium oxide

Kc Kilocycles

lb Pound

M100 Modulus at 100% elongation

M300 Modulus at 300% elongation

MgO Magnesium oxide

MH Maximum torque

mm milimeter

mm/min Millimeter per minute

Na2O Sodium oxide

-NH- Amine

-OH Hydroxyl group

Pd-Au Palladium-Aurum

xiii

phr Part per hundred rubber

SiO2 Silica (silicon dioxide)

t90 Cure time

ts2 Scorch time

T5 Decomposition temperature at 5% weight loss







TiO2 Titania (titanium dioxide)

wt % Weight percentage

xiv

LIST OF ABBREVIATION

Abbreviation Description

ASTM American standard testing and materials

BKF 2, 2 methylene-bis-(4-methyl-6-tert-butylphenol)

BR Polybutadiene rubber

CB Carbon black

CBS N-cyclohexyl-2-benzothiazole sulfenamide

CEC Cation-exchange capacity

CNT Carbon nanotubes

C.V. Continuous vulcanization processes

CV Conventional vulcanization systems

DPG Diphenyl guanidine

EPC Easy processing furnace

EV Efficient vulcanization systems

ENR Epoxidized natural rubber

ENR 25 Epoxidized natural rubber with 25% mol of epoxy

ENR 50 Epoxidized natural rubber with 50% mol of epoxy

EPDM Ethylene-propylene-diene

FEF Fast extruding furnace

FTFT Fatigue-to-Failure Tester

FT Fine thermal

HAF High abrasion furnace

HMF High modulus furnace

HNTs Halloysite nanotubes

IR Polyisoprene

xv

ISAF Intermediate SAF

J.I.S Japanese industrial standard

LNR Liquid natural rubber

MBT Mercaptobenzothiazole

MBTS Dibenzothiazyl disulfide

MMT Montmorillonite

MT Medium thermal

NR Natural rubber

OENR Oil-extended natural rubber

PET Polyethylene terephthalate

PPD Para-phenylenediamine

PRI Plastic retention index

RRIM Rubber Research Institute Malaysia

SAF Super abrasion furnace

SBR Styrene-butadiene rubber

SEM Scanning electron microscope

Semi-EV Semi-efficient vulcanization systems

SMR Standard Malaysian Rubber

SRF Semi-reinforcing furnace

TBBS N-tert-Butyl-2-benzothiazole sulfenamide

TGA Thermogravimetric analysis

TPG Triphenyl guanidine

TSR Technically specified grades

ZnO Zinc oxide

xvi

NANOKOMPOSIT TIUBNANO HALLOYSITE/GETAH ASLI (HNTs/NR):

PENYEDIAAN DAN SIFAT-SIFAT

ABSTRAK

Kajian ini tertumpu tentang kesan pelbagai pembebanan HNTs terhadap sifat-sifat

nanokomposit getah asli. Siri (1), HNTs diisi di dalam dua jenis getah asli iaitu tidak

diubahsuai dan ubahsuai yang masing-masingnya dinamai SMR L dan ENR 50.

Dalam Siri (2), kajian mengenai penggantian separa HNTs dengan karbon hitam

(CB) sebagai pengisi hibrid di dalam getah asli dikaji. Akhir sekali dalam Siri (3),

kajian mengenai perbezaan jenis kaedah pencampuran iaitu kaedah pencampuran

mekanikal dan larutan telah dibezakan. Sistem pemvulkanan konvensional (CV)

telah dipilih dalam kajian ini dan penggiling bergulung dua makmal digunakan untuk

proses pencampuran. Sifat-sifat yang dikaji dalam kajian ini termasuk ciri-ciri

pematangan, sifat-sifat mekanikal, peratusan pembengkakan, hayat fatig, kestabilan

termal dan sifat-sifat morfologi getah asli yang diisi oleh HNTs. Mosanto Moving

Die Rheometer (MDR) dengan suhu 150°C telah digunakan untuk menentukan ciri-

ciri pematangan nanokomposit. Dalam Siri (1), kekuatan tensil optimum

nanokomposit HNTs/SMR L dan HNTs/ENR 50 dicapai dengan 20 phr. Kestabilan

termal meningkat dengan penambahan HNTs di dalam kedua-dua nanokomposit.

Dalam Siri (2), kekuatan tensil, modulus tensil, dan kestabilan termal nanokomposit

meningkat. Dalam Siri (3), nanokomposit kaedah pencampuran larutan menunjukkan

kekuatan tensil, modulus tensile dan hayat fatig yang lebih tinggi pada mana-mana

pembebanan HNTs. Amnya, imej SEM menunjukkan pembebanan HNTs yang

rendah boleh tersebar secara seragam di dalam matrik tetapi penyebaran yang teruk

dan kurang memuaskan telah diperhatikan pada pembebanan HNTs yang lebih

tinggi.

xvii

HALLOYSITE NANOTUBES/NATURAL RUBBER (HNTs/NR)

NANOCOMPOSITES: PREPARATION AND PROPERTIES

ABSTRACT

This research focused on the effect of various HNTs loading on the properties of

natural rubber nanocomposites. Series (1), HNTs filled in two types of natural rubber

i.e., unmodified and modified natural rubber namely SMR L and ENR 50

respectively were investigated. In Series (2), the study of partial replacement of

HNTs with carbon black (CB) as hybrid fillers was studied. Lastly in Series (3), the

study regarding different type of mixing methods i.e., mechanical and solution

mixing methods were compared. Conventional vulcanization (CV) system has been

selected in this research and laboratory two-roll mill was used for mixing process.

The studied properties in this research includes curing characteristics, mechanical

properties, swelling percentage, fatigue life, thermal stability and morphological

properties of HNTs filled natural rubber. Mosanto Moving Die Rheometer (MDR

2000) with temperature at 150°C was used to determine curing characteristics for

nanocomposites. In Series (1), optimum tensile strength of HNTs/SMR L and

HNTs/ENR 50 nanocomposites was achieved with 20 phr. Thermal stability

increased with addition of HNTs in both nanocomposites. In Series (2), tensile

strength, tensile modulus and thermal stability of nanocomposites increased. In

Series (3), solution mixing method nanocomposites showed higher tensile strength,

tensile modulus and fatigue life at any HNTs loading. Generally, the SEM images

showed that lower HNTs loading can be dispersed uniformly in matrix but bad and

poor dispersion was observed at higher HNTs loading.

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

The discovery regarding the reinforcement effect in rubber properties by

adding certain additives called filler can be considered the most important finding in

rubber technology after the discovery of vulcanization process. This turned natural

rubber vulcanizates into more desirable properties. Basically, it is started with the

usage of zinc oxide (ZnO) for its color at the early years of 20th

century and in the

same time it is discovered that ZnO also showed their reinforcement potential (Ciullo

and Hewitt, 1999). Then, continued with the usage of carbon black (CB). Up until

now, CB still the most important filler in industry (Mouri, 2001) and ZnO still being

used in rubber compound since it is accidently found that ZnO is suitable used as

activator (Ciullo and Hewitt, 1999).

Afterward, the studies continued with introduction of nonblack filler into

rubber compound. Generally, the addition of nonblack filler is to produce white or

light colored products which cannot be produced by using CB. The most three

nonblack fillers that widely used are calcium carbonate (CaCO3), clay and

precipitated silica which also showed improvement in rubber properties

(http://www.rtvanderbilt.com/NonBlackFillers.pdf).

Many researchers show great interest to investigate potential of other

materials which may have possibility to act as filler as good as CB or silica in rubber

compounds. Therefore, wide varieties of materials generally from renewable

2

resources such as agricultural waste, recycled products, and also from mineral

resources or from other resources have been investigated tremendously for this

purpose. For examples, rice husk ash (Ismail et al., 1998), oil palm fiber (Ismail et

al., 1997) or starch (Wu et al., 2009) from agricultural waste while recycled

polyethylene terephthalate (PET) bottles (Nabil et al., 2011) from recycled products

and various types of clay such as montmorillonite (MMT) (Joly et al., 2002), kaolin

(Yahya et al., 2009), bentonite (L‟alikova et al., 2010) from mineral resources has

been used lately. These materials not only limited in rubber composites but also in

other polymer matrixes.

Then recently, considerable attempt has been expanded in rubber

nanocomposites studies due to revolution from micro-sized filler to nano-sized

fillers. The switch using micro-sized filler to nano-sized filler due to several

advantages displayed by nano-sized filler filled rubber. Some of the advantages of

nanocomposites are improvements in the mechanical properties, heat stability, flame

retardance, gas-barrier properties, abrasion resistance, and solvent resistance (Wu et

al., 2001). It can be related to high aspect ratio and the dispersion of filler in nano-

dispersion where it can be expected better interfacial between filler and rubber which

is fundamentally different compared to microcomposites (Maiti et al., 2008).

Besides, reinforcement is possible at very low loading of fillers (< 10 wt %) if

compared to conventional filler composites, which require much larger quantities

(Teh et al., 2004a).

Currently, nanoclay loaded rubber nanocomposites is one of fast growing

development which has been studied in various fields of study for examples in

3

mechanical, thermal or electrical purposes. Clay minerals are potential nanoscale

additives because of their layers consisting of 1 nm thick planar structures (Kato et

al., 2006). Toyota group is the first group who utilized silicate layered clay namely

montmorillonite (MMT) in polyamide compound and demonstrated significantly

increased in modulus, tensile strength and heat distortion temperatures (Ismail et al.,

2008). The result obtained made other researchers eagerly to explore more about

MMT and similar materials in natural rubber properties.

1.2 Problem statement

It is well known that natural rubber is the most popular rubber being used in

industry due to their unique properties. Usually, natural rubber is loaded with fillers

to modify the properties of rubber vulcanizate, to reduce cost of production or to

impart the color.

In this work, HNTs is loaded into natural rubber since no details works has

been done. Rooj et al. (2010) only show basic information about the addition of

HNTs inside natural rubber. Thus, this study is important to know in more details

about the possibility affect of HNTs loading towards natural rubber properties.

However, some of limitation suffered by natural rubber is due to non-polar

nature property. Natural rubber showed weakness in making good interaction with

filler due to this reason. Thus, epoxidized natural rubber with 50% mol of epoxy is

used as matrix for a comparison with unmodified natural rubber. The polar property

in ENR 50 is expected to interact with HNTs and consequently show positive effect

in the properties of HNTs/ENR 50 nanocomposites. Sadequl et al. (1999) observed

4

CB and silica filled ENR 25 matrix able to show better interaction than CB or silica

filled natural rubber where the tensile strength of ENR composites is higher than

natural rubber composites but the fatigue life of natural rubber composite is higher

than ENR 25 composites.

Various studies investigated the effect of hybrid fillers in natural rubber

composites. This is to investigate the possibility of synergism effect by well known

reinforced filler and new fillers. Recently, hybrid polymer nanocomposites

containing layered silicate and with well-known reinforced filler gained huge

attention. For example, Chattopadhayay et al. (2010) used CB/organoclay hybrid

filler in ENR 25 nanocomposites. It is found that the addition CB/organoclay with

ratio 20:15 show synergistic effect reflected in their dynamic mechanical and tensile

properties.

Filler dispersion is one of the critical factors that influence the properties of

final rubber composites. Hence, the mixing process plays the most important step in

determine good dispersion of filler inside rubber matrix. Generally, there are several

types of mixing methods such as melt mixing, solution mixing, and also in situ

mixing methods that commonly used. Besides melt mixing, solution mixing is the

easiest mixing methods and usually been chosen by many researchers in order to

obtain better filler dispersion in rubber composites. Ismail et al. (2010a) studied two

types of mixing methods for carbon nanotubes (CNT)/natural rubber

nanocomposites. From this study, they concluded that solution mixing method can

exhibited better mechanical properties than nanocomposites prepared by mechanical

mixing method due to well dispersed of CNT in natural rubber matrix for solution

5

mixing method. Lopez-Manchado et al. (2004) compared the properties of

organoclay-natural rubber nanocomposites prepared by mechanical and solution

mixing where they obtained similar trend as reported by Ismail et al. (2010a).

1.3 Research objectives

This research is study about the effect of various HNTs loading on properties

of natural rubber. Then, it is expanded into several parameters. The main objectives

of this research are:

1. To compare and investigate the effect of HNTs loading on properties of

different types of rubber matrix which is between SMR L and ENR 50.

2. To investigate the effects of HNT/CB hybrid fillers in natural rubber

nanocomposites.

3. To study the effect of different mixing methods to prepare HNTs filled

natural rubber nanocomposites.

1.4 Scope of research

The scope of this research is summarized in Figure 1.1. In this figure, the

preparation to produce natural rubber nanocomposites filled with HNTs starts with

the preparation of filler. The next step is mixing procedure by using external mixing

methods, laboratory two-roll mill. Then, the uncured samples are tested by Moving

Die Rheometer (MDR 2000) to determine the curing characteristics of samples. After

that, the cure times obtained were used in compression molding. Next step is to

examine and study the properties of filled natural rubber in tensile properties,

swelling percentage, fatigue life and also thermal stability. Lastly, the morphological

6

properties of tensile and fatigue fractured surface is investigate by scanning electron

microscope (SEM). Then, overall results are analyzed and discussed.

HNTs preparation

(sieved and dried)

Mixing process

(two-roll mill)

Curing test

(Monsanto Moving Die Rheometer, MDR 2000)

Compression molding

(hot press)

Testing:

- Tensile properties,

- Swelling percentage,

- Fatigue life,

- Thermal stability

Scanning Electron Microscope (SEM):

- Tensile,

- Fatigue fractured surface

Figure 1.1 Summary for scope of research in this study.

7

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction to rubber

There are many definitions for rubber can be found. Term of elastomers also

come along when it‟s regarding about rubber. Basically, term „rubber‟ originally

meant the materials obtained from the rubber tree Hevea brasiliensis while elastomer

is preferred term for vulcanized rubber (Nagdi, 1993). Generally, Lesko (2008)

explained that rubbers and elastomers are materials that return to their original shape

after being stretched to several times their length.

However, in more details, rubber and elastomer can be differentiated on the

basis of how long a deformed material requires to return to its approximately original

size after deforming force is removed and by its extent of recovery (Rosato et al.,

2000). Rubber is defined as a material that is capable of recovering from large

deformation quickly and forcibly, and which in its modified state, retracts within one

minute, to less than 1.5 times its original length after being stretched at room

temperature to twice its length and held for one minute before release. An elastomer

is defined as a macromolecular material which, at room temperature, is capable of

recovering substantially in shape and size after removal of deforming force, with no

time is given for full recovery (Lesko, 2008).

Based on the general definition, various materials which show similar

behavior just aforementioned can be classified into two main groups, i.e., natural and

synthetic rubber (Chandrasekaran, 2007). Natural rubber is polyisoprene extracted

8

from Hevea brasiliensis (Ciesielski, 1999). While, synthetic rubber may be defined

as one built up by chemical means from various substances, and possessing all the

chemical and physical properties of natural rubber (Wright, 1908).

Natural rubber exhibits a set of properties which unequalled by synthetic

rubber such as higher elasticity, strain-induced crystallization, low heat build-up and

also high tack and green strength. High elasticity of natural rubber is owing to it‟s

crystallize ability under strain (100% cis- and associative structure). Strain-induced

crystallization is largely due to the stereochemical regularity of macromolecules

(natural rubber: 100% cis as shown in Figure 2.1) and such crystallization is

reversible, with crystallite disappearing when stretching stops. Moreover, natural

rubber is the elastomers with the lowest heat build-up making it unavoidable for

dynamical applications with high strain and high frequencies such as tires subjected

to heavy strains. Furthermore, high inherent tack and green strength are the most

important aspects of processablity of natural rubber. The mechanism responsible for

high green strength is strain crystallization which it not active in the bond formation

stage and hence does not interfere with contact and interdiffusion, but rather

develops upon stretching. High tack of natural rubber is attributed to its great ability

to flow under compressive load and its high green strength (Mathew, 2007).

Figure 2.1: General chemical structure of cis-1, 4-polyisoprene (natural rubber).

9

The exploration of natural rubber expends all the applications in the human

necessities. Initially, natural rubber has been used in production of tires widely and

any kind of rubber applications. But then, there are many attempts to produce

synthetic rubber in order to replace natural rubber. Synthetic rubber production is

started during the World War I in German due to inadequate natural rubber from the

tropical regions where it produced. Similarly, the massive American effort of World

War II was driven by the shortage of natural rubber and accelerated when the

Japanese take control of Asian plantation areas (Henderson, 1987).

Currently, synthetic rubber has been conquered around 60% market share of

the world‟s total consumption. The introduction of synthetic rubber has also capped

the increase in the prices of natural rubber but reduced the volatility of price

fluctuation (Lim, 2004). This definitely due to better properties of synthetic rubber

compared than natural rubber. For instance, polyisoprene (IR) is an excellent choice

when processing is important while tack and green strength are not critical. IR is

gaining favor in medical applications as replacement for natural rubber to eliminate

the dangers of patients suffering allergic reactions to the natural proteins and

impurities found in natural rubber. Ethylene-propylene-diene (EPDM) is the most

widely used when superior resistance to ozone is required. Besides, EPDM also can

be highly loaded with low-cost filler, including clays, silica, and carbon black (CB)

and talc (School, 2001).

However, due to increase the awareness of renewable and environmental

friendly resources caused renewed interest in the development of modified natural

rubber (Mathew, 2007). This is because synthetic rubber is obtained from the

10

petroleum refineries as a by product (Brahman, 2002). Generally, rubber can be

produced from petroleum because both are members of vast chemical family of

hydrocarbons – carbon and hydrogen in combined form (Dherer, 1942). Thus,

exploration modified natural rubber as new choice to replace synthetic rubber is

begin.

2.1.1 Natural rubber

There are two generic classifications of natural rubber, the crepes and sheets,

and the technically specified grades (TSR) (Day, 2001). The conventional forms of

natural rubber, the crepes and sheets, are graded according to the “Green Book”,

which sets out the International Standards of Quality and Packing for Natural Grades.

Grading is by visual examination and is based on the presence or absence of

extraneous foreign matter (dirt), bubbles, uniformity and intensity of color, mold and

rust spots, and so on. Low dirt content generally reflected for better technological

properties but most of the other visual criteria have no technological basis

(Subramaniam, 1987).

The introduction of TSR by Standard Malaysian Rubber (SMR) scheme in

1965 is a major step forward in providing natural rubber to guaranteed specifications

(Elliot, 1979). TSR are different from crepes and sheets in several aspects, most

noticeably in weight of TSR rubber supplied which is in 75 lb bales rather than 250

lb blocks. Moreover, TSR is not graded visually, but by chemical test (Day, 2001).

The advantages of TSR include assurance of quality for important technical

parameters, consistency in quality, and minimum space for storage and clean and

11

easy to handle packing (Mathew, 2001). Important TSR grades are shown in Table

2.1 (Rodgers and Waddell, 2005).

Table 2.1: Technical Grades of Natural Rubber (Rodgers and Waddell, 2005).

Property SMR

L

SMR

CV

SMR

5

SMR

10

SMR

20

SMR

50

Dirt (% maximum) 0.05 0.05 0.05 0.10 0.20 0.50

Ash (% maximum) 0.60 0.60 0.60 0.75 1.00 1.50

Volatile matter

(%)

0.80 0.80 0.80 0.80 0.80 0.80

Nitrogen (%) 0.60 0.60 0.60 0.60 0.60 0.60

Plasticity 30.00 - 30.00 30.00 30.00 30.00 1PRI, % 60.00 60.00 60.00 50.00 40.00 30.00

Color index 6.00 - - - - -

Mooney Viscosity - 60.00 - - - - 1 PRI: Plastic retention index

As can be seen in Table 2.1, all grades show lowered limit for nitrogen and

volatile matter. The volatile matter is limited to a maximum 0.80% which is an

assurance that the rubber is adequately dried and this should be reduce, if not

eliminate, the occasional bale that is found contain wet patches, which has been a

sporadic problem in the past. Plasticity retention index (PRI) is a measure of raw-

rubber oxidisability, high values indicating greatest resistance to oxidation (Elliot,

1979).

2.1.2 Epoxidized natural rubber (ENR)

Modified natural rubbers can be done by physical and/or chemical means

which resulted in several suitable forms for specific processes and applications.

These included viscosity-stabilized rubber, thermoplastic natural rubber, oil-extended

natural rubber (OENR), epoxidized natural rubber (ENR), graft copolymers and

chlorinated rubber and liquid natural rubber (LNR) (Kurian and Peter, 2007).

Modified natural rubber products have attained some applications such as chlorinated

12

rubber is used as an adhesive for metal to rubber bonding and for chemically

resistant paints while ENR is the most attractive from both a technical and economic

point of view (Feldman and Barbalata, 1996).

The most important modification of natural rubber is perhaps ENR. ENR can

be produced by epoxidizing natural rubber in the latex stage by reaction with formic

acid and hydrogen peroxide. It may called as ENR 10, ENR 25, and ENR 50 which

contain 10 mole %, 25 mole % and 50 mole %, respectively, of epoxide groups

(Subramaniam, 1987). Commercially, there are two forms of ENR available in

market which is ENR 25 and ENR 50 (Liu, 1999). General structure of ENR is

shown in Figure 2.2 (Karak, 2009).

Figure 2.2: General chemical structure of epoxidized natural rubber (ENR) (Karak,

2009).

The modification of natural rubber by replaced unsaturated backbones with

epoxy groups progressively shows improvement in certain properties. ENR has

unique set of properties which offering high strength, due to ability to undergo strain

crystallization, along with increased glass transition temperature and solubility

parameters. These properties are reflected as increased oil resistance, enhanced

adhesive properties and a high degree of damping and reduced gas permeation

(Gelling, 1991).

13

Besides that, filled ENR has been popular research subject due to the

versatility of ENR to accept numerous types of fillers. For instance, silica filled ENR

has show significant improvement in tensile and tear strength without using coupling

agent (Mohd Ishak and Bakar, 1995).

2.2 Rubber compound

Traditionally, a typical manufacturing sequence comprises mixing, forming,

and vulcanizing in the rubber manufacturing industry. The compounding of rubber

products starts with the choice of elastomers, filler (filler or extending), crosslinking

chemical, and various additives which, when mixed together, will provide a

compound with the desired properties and performance (Ciullo and Hewitt, 1999).

Table 2.2 shows the proportions of various ingredients for most rubber products

where the base rubbers are considered as 100 parts by weight (Chandrasekaran,

2007).

Table 2.2: Various ingredients in rubber compounds (Chandrasekaran, 2007).

Ingredient Parts per hundred rubber (phr)

Base rubber or blend of rubbers 100.00

Vulcanizing agent (sulfur) 0.50 – 40.00

Accelerator 0.50 – 5.00

Activator 1.00 – 5.00

Antioxidant 0.50 – 2.00

Reinforcing fillers, carbon black, and minerals 25.00 – 200.00

Processing oils 0.00 – 25.00

Inert fillers 25.00 – 200.00

Coloring additives 1.0 – 5.00

2.2.1 Mixing process

Mixing is defined as a process which reduces the composition nonuniformity

or inhomogeneity of a mixture of two or more components (Gupta, 1998). In general,

four unit processes are involved in mixing process, each requiring separate

14

consideration; mastication, incorporation, dispersion and distribution (Nakajima,

2000). The first step in rubber compounding is mastication or polymer “breakdown”

(Ciullo and Hewitt, 1999). During mastication, natural rubber chains break, resulting

in a substantial decrease of molecular weight (Nakajima, 2000). The viscosity of

natural rubber must be reduced to produce good mixing and processing (Martin,

1997).

Then, incorporation is a process where filler particles become included inside

the rubber. The rubber fills the voids space and cavities in the filler agglomerate

where rubber-particle complex acts as a single deformable unit under the application

of shear in mixing equipment. Dispersion is a process where filler such as carbon

black pellets and their fragments, agglomerates are broken down to the primary units

called aggregates. If the filler particles do not get incorporated into the rubber matrix,

its dispersion cannot be achieved. Distribution corresponds to mixing in the usual

sense except that it is homogenization on the macro-scale (Nakajima, 2000).

These steps are accomplished essentially trough the application of very high

shear force and pressure through a specific design of mixers (Gupta, 1998). The most

common equipment used for mixing process are two-roll mills, internal batch

mixers, continuous mixers, extruders or combination thereof (Melotto, 1997).

Definite time, temperature range and sequence schedule are important factors

when using two roll mill as mixer. Besides, the space between the rolls is increased

at intervals to maintain a constant bank. In addition, powders that drop into the mill

tray are swept to the front by the operator and added back to the mix. During the

15

mixing operation, cutting and blending is carried out in order to obtain a thorough

and uniform dispersion of the ingredients in the rubber mix (Nagdi, 1993).

2.2.1 Forming

After mixing, the compounded rubber is plastic and is now ready to be

formed into a shape for vulcanization. The most common process is by molding

under pressure in a heated molding such compression, transfer and injection molding

(Thomas and Stephen, 2010). The other methods are extrusion or calendering.

In compression molding, mixed compound is added in the form of slabs

directly into a mold, which is subsequently closed to shape the desired part. In

injection molding, a rubber strip is fed into a chamber or reciprocating screw which

meters and injects the rubber compound into a mold. Transfer molding combines

features of both (White, 1995). Extrusion is used to form final product in the case of

hoses, tubes and profiles, and many tire components are extruded before being built

into a tire. The extruder itself has two main functions: to pump the rubber compound

through the barrel and to generate enough pressure in the process to force the

material through a die to give the required cross-sectional shape. Calendering

machines are used to produce continuous sheets from rubber compounds, sometimes

incorporating reinforcing materials such as textile or wire cord, and for impregnating

or coating fabrics with compound (Johnson, 2001).

2.2.2 Vulcanization

Sulfur vulcanization takes places in three stages: induction, curing and

reversion or overcure (Martin-Martinez, 2002). Induction period is the time elapsed

16

before crosslinking starts. Sufficient delay or scorch resistance in needed to permit

mixing, shaping and forming, and flowing in the mold before vulcanization. Scorch

resistance is usually measured by the time at a given temperature required for the

onset of crosslink formation as indicated by an abrupt increase in viscosity (Coran,

2005). At the second stage, cure time is the time required for the compound to reach

a state of cure where the desired balance of properties has been attained. When a

compound is cured beyond the point where its balance of properties has been

optimized it becomes overcured. For most elastomers, overcure means the compound

becomes harder, weaker and less elastic (Hewitt, 2007).

During vulcanization, the changes may occur is long chains of rubber

molecules become crosslinked by reactions with the vulcanization agent to form

three-dimensional structures where soft, weak plastic-like material transform into

strong elastic product. In addition, rubber loses its tackiness and becomes insoluble

in solvents and is more resistant to deterioration which normally caused by heat,

light, and aging processes (Stephens, 1987).

Stephens (1987) outlined eight significant methods for vulcanization

techniques which are compression molding, transfer molding, injection molding,

open cure, continuous vulcanization processes (C.V.), cold vulcanization, high-

energy radiation and microwave vulcanization. However, there are three important

variants of the methods: compression, transfer and injection where compression mold

is the most widely used. Basically, the vulcanizing process applied heat at an

elevated temperature for a given time to cure the product so it takes up a shape in

mold (Ma et al., 2009).

17

2.3 Compounding ingredients

As aforementioned, rubber products greatly depend on the types of rubber

used as matrix. However, the properties of rubber products also can be modified

through appropriate selection of compounding ingredients. Careful selection of

compound ingredients may show major impact in final properties. Hence, various

functions of compounding ingredients are required such as activator, accelerator,

vulcanization agent, antidegradant, and miscellaneous ingredients.

2.3.1 Vulcanization agents

Materials that are able to form crosslinks between polymer chains may be

generally classified as vulcanizing agents (Datta and Ingham, 2001). Vulcanization

converts a substance that is plastic and moldable into one that is flexible and elastic

(Ciullo and Hewitt, 1999).

There are several types of vulcanization system used in rubber industry

dependent on the rubber used. Vulcanization agent can be grouped into two main

groups: sulfur and related elements and nonsulfur vulcanization. Sulfur exists in two

forms, rhombic and amorphous (or insoluble). Rhombic form is normally used for

vulcanization where it exists as a cyclic (ring) structure composed of eight atoms of

sulfur while the amorphous form is actually polymeric in nature; it is a metastable

high polymer. In addition, two other elements in the same periodic family as sulfur,

namely selenium and tellurium also capable of producing vulcanizations (Stephens,

1987).

18

For sulfur system, there are three main systems in terms of the usage of sulfur

in the vulcanized network namely conventional (CV), efficient (EV) and semi-

efficient (Semi-EV) vulcanization systems as shown in Table 2.3 (Datta, 2001). For

EV system, low or even zero level of sulfur with high level of accelerator and

possibly a sulfur donor is used. In this system, resultant natural rubber vulcanizates

has a high proportion of mono- and disulphide crosslinks and low degree of main-

chain modifications giving high resistance to thermal and oxidative ageing. While

the conventional system show natural rubber vulcanizates with high proportion of

main-chain modifications. This gives poor resistance to thermal and oxidative ageing

(Rodger, 1979). High sulfur level and low accelerator concentration used in CV

system forming polysulfidic crosslinks in natural rubber vulcanizates and thermally

unstable and readily oxidized. However, have better fatigue resistance. In Semi-EV

system, which are intermediate between EV and CV system, are a compromise

between resistance to oxidation and required product fatigue performance (Rodgers

and Waddell, 2005).

Table 2.3: CV, Semi-EV and EV vulcanization systems 3 (Datta, 2001).

Type Sulfur (S, phr) Accelerator (A, phr) A/S ratio

CV 2.0 - 3.5 1.2 – 0.4 0.1 – 0.6

Semi-EV 1.0 – 1.7 2.4 – 1.2 0.7 – 2.5

EV 0.4 – 0.8 5.0 – 2.0 2.5 – 12

Generally, sulfur is the most commonly used as curing agent for unsaturated

rubber, particularly for diene rubbers such as natural rubber, styrene-butadiene

rubber (SBR), polybutadiene, nitrile, polychloroprene, and polyisoprene and not

suitable for saturated rubber. While, alternate curing system is required for

elastomers with chemically saturated backbones because they cannot be crosslinked

with sulfur (Ciullo and Hewitt, 1999). The system used for saturated elastomer is

19

called nonsulfur vulcanization agents. Most nonsulfur vulcanization agents belong to

one of three groups: metal oxides, difunctional compounds or peroxides (Stephens,

1987).

2.3.2 Activators

The rate at which sulfur reacts with unsaturated polymer can be accelerated

by activators: a metal oxide plus fatty acid. The most common combination is zinc

oxide (ZnO) and stearic acid, with the fatty acid solubilizing the zinc in the

elastomers. Zinc oxide is inorganic activator (lead and magnesium oxides are alos

used, but less often), while stearic acid is the organic activator (To, 2001). It is

believed that the sulfur reacts, in the presence of metal, as a cation at the double

bond. This results in charged and uncharged polysulfides, the latter of which could

form free radicals. Metal activated vulcanization will proceed more rapidly than

crosslinking by sulfur alone, but still too slow for most production purposes. The

metal oxide/fatty acid is, in practice, used not to activate the sulfur itself, but to

activate the organic compounds used as vulcanization accelerators (Ciullo and

Hewitt, 1999). In other words, activators help accelerators in vulcanization process

(Chandrasekaran, 2007).

2.3.3 Accelerators

Accelerators are products which increase both the rate of sulfur crosslinking

in a rubber compound and crosslink density (Rodgers and Waddell, 2005). This

indicates function of accelerator not only to speed up the vulcanization, but also to

increase the efficiency of vulcanization by encouraging useful mono- and disulphide

crosslinks to form, rather than to waste sulfur in forming ineffective polysulfide

20

crosslinks and cyclic structures (Alger, 1997). The choice of accelerator will affect

the scorch (premature vulcanization) safety, the cure rate, and the length and number

of crosslinks which form (Ciullo and Hewitt, 1999).

There are many accelerators available which has been grouped into several

classes. Accelerators may be classified in several ways: inorganic or organic, acidic

or basic, by chemical types or by speed of the cure, giving rise to the terms slow,

medium, semi-ultra and ultra (Datta, 2001).

Most accelerators fall into one of eight groups: aldehydeamines, thioureas,

guanidines, thiazoles, sulfenamides, dithiocarbamates, thiurams and xanthates

(Rodger and Waddell, 2005). Accelerator in the same group may exhibit similar or

different speed of cure. For instance, diphenyl guanidine (DPG) and triphenyl

guanidine (TPG) are in the same group but DPG show medium accelerator while

TPG show slow accelerator activity. Mercaptobenzothiazole (MBT) and

dibenzothiazyl disulfide (MBTS) from thiazoles group also act differently where

MBT show semi-ultra accelerator while MBTS exhibit semi-ultra (delayed action)

accelerator (Ghosh, 2002).

2.3.4 Antidegradants

An antidegradant is a compounding material used to retard deterioration

caused by oxidation, ozone, light or combinations of these (Ciesielski, 1999).

Unsaturated elastomers such as NR, SBR, polybutadiene rubber (BR), nitrile rubber

and neoprene usually need antidegradants. This is because unsaturated elastomers

containing unsaturated bonds with allylic or tertiary benzylic hydrogen atoms are

21

somewhat reactive and more prone to degradation. The degradative results may due

to chain scission which caused loss of physical properties, or increased in stiffness

and hardness due to extra crosslinking formation, or catastrophic failure in fatigue

cracking during dynamic flexing and/or surface cracking and crack growth when

rubber is under stress particularly when stretched either under static or dynamic

conditions (Ignatz-Hoover, 2001).

There are two main classes of antidegradants used in rubber which are

antioxidants and antiozonants. Antioxidants belong to the class of compounding

ingredients known as protective agents, responsible to remove free radicals which are

generated by the interaction of oxygen and the polymer at elevated temperature

(Simpson, 2002). Oxidation is a cyclic free radical chain process that proceeds by

two mechanisms: chain scission of the polymer backbone causes softening and

weakening while radical-induced crosslinking causes hardening and embrittlement.

For instance, chain scission is the primary mechanism in natural rubber while

crosslinking is predominant with styrene-butadiene rubber (SBR) (Ciullo and Hewitt,

1999). Hence, the addition of antioxidant acts either by interrupting chain reactions

or by preventing free-radical formation (Fishbein, 1983). The chemical structure of

2, 2 methylene-bis-(4-methyl-6-tert-butylphenol) (BKF) is shown in Figure 2.3.

Figure 2.3: Chemical structure of 2, 2 methylene-bis-(4-methyl-6-tert-butylphenol)

(BKF).

22

Antiozonants is an ingredient to protect rubber compound against the

deteriorating influence by ozone, the function of antiozonants is not limited to

protection against ozone attack, but also give high protection against oxidative and

thermal degradation, and against fatigue failure (flex cracking) (Simpson, 2002). The

most widely accepted class of antiozonants is derivatives of para-phenylenediamine

(PPD) and general structure is shown in Figure 2.4 (Birdsall et al., 1991).

Figure 2.4: General structure of para-phenylenediamine (PPD).

2.4 Fillers

The use of fillers in rubber products is nearly as old as the use of rubber itself

(Ciullo and Hewitt, 1999). Wide variety of fillers is added into rubber matrix in order

to modify the properties of rubber. It can be to improve cured physical properties,

reduce cost, and/or impart color to the rubber products (Waddell and Evans, 2001).

Fillers can be divided into three main groups; reinforcing, semi-reinforcing or

non-reinforcing fillers (Sae-Oui et al., 2009). Reinforcing fillers are ingredients

added which increase stiffness, hardness, strength of rubber and its resistance to

abrasion, cutting and tearing. Their effect in rubber is not chemical but physical

depending mainly on the fineness and the homogeneous dispersion in rubber. Carbon

black, zinc oxide, magnesium carbonate, silica and China clay belong to the

reinforcing class of filler (Chandrasekaran, 2010). Semi-reinforcing filler is a

particulate material that is able to increase tensile strength and tear strength but does

23

not improve abrasion resistance (Skelhorn, 2003). Fillers that only lead to small

increases in viscosity of the compound, cause deterioration of the mechanical

properties of the vulcanizates, and do not exhibit any reinforcing action are called

non-reinforcing or inactive fillers (e.g., calcium silicate, chalk powder, etc) (Kim et

al., 2011).

Filler properties that have the most influence on rubber processing and

vulcanization are particle size, surface area and character, and structure (Arayapranee

and Tempel, 2008). Generally, smaller particulate filler impart greater reinforcement.

Besides, particle with a high aspect ratio has higher reinforcement than a more

spherical one (Wagner, 2011). Polymer-filler bonding can be physical or chemical

interaction. These interactions such carbon black in rubber lead to formation of high

modulus compound even in the absence of coupling agent which is a very clear

indicator that polymer-filler bonding has taken place. Particle shape may provide

further changes in reinforcement. Overall, the effect on reinforcement is relatively

small. Particle complexity has more pronounced effect on processing behavior than

on reinforcement and provides important benefits in this area (Skelhorn, 2003).

2.4.1 Halloysite nanotubes (HNTs)

Clay minerals may be liberated by the breakdown of geological mudstones

and clays. However, in natural soils, most are the products of chemical weathering.

Clay minerals are composed of hydrated aluminium, iron or magnesium silicates that

are combined in complex crystalline structures. The main building blocks of the clay

minerals have been portrayed as layers of silica tetrahedral, involving silicon cations

and 4 oxygen anions and aluminium octahedra involving 6 hydroxyl or oxygen

24

anions and cations of aluminium, or sometimes magnesium, calcium etc (Haigh,

2000).

Clay minerals, on the basis of their crystalline arrangement, can be divided

into three main groups. It is observed that more or less similar engineering properties

are exhibited by all clay minerals belonging to a particular group. The group of

minerals and the most important clay minerals in each of these groups can be seen in

Table 2.4 (Ranjan and Rao, 2007).

Table 2.4: Clay minerals (Ranjan and Rao, 2007).

Name of mineral and group Structural formula

Kaolin group

a) Kaolinite (OH)4.Al4Si4O10

b) Halloysite (OH)4.Al4Si4O10.4H2O

Montmorillonite group

Montmorillonite (OH)4.Al4Si4O20.nH2O

Illite group

Illite (OH)4Ky(Sig.yAly)(Al4.Mg6.Fe4.Fc6)O20

As can be seen in Table 2.4, halloysite is one of clay in kaolin group just like

kaolinite. Halloysite is defined as two layered aluminosilicates, chemically similar to

kaolin, which has predominantly hollow tubular structure in the submicron range.

Interlayer water in HNTs is one of the most important features distinguish HNTs

from kaolinite (Du et al., 2010). The physical appearance of HNTs is very similar to

that CNTs, but the cost of HNTs is much lower (Ng et al., 2011). Figure 2.5 show

typical schematic diagram for HNTs. Beside, HNTs may exist in two forms. Firstly,

may exist as hydrated halloysite with basal spacing (d001) of 10Å. However, due to

interlayer water weakly held, so that halloysite (10Å) can readily transform to

halloysite (7Å) which also known as methahalloysite (Yuan et al., 2008).

halloysite nanotubes/natural rubber (hnts/nr...

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